The present invention relates generally to subscription television systems and, more particularly, to an improved technique for encrypting a transmitted mode control signal which identifies, for example, whether the video fields of a subscription television signal are being transmitted in a normal or in an inverted form.
In subscription television systems, television signals are typically transmitted to system subscribers in a scrambled form either along a coaxial cable or as an "over-the-air" broadcast. Each system subscriber is provided with a decoder operable for unscrambling the transmitted television signal and for supplying the unscrambled signal to a conventional television receiver for viewing. In addition, many current subscription television systems are addressable in nature whereby the operation of individual decoders may be selectively controlled through the transmission of system control data generated at the system head-end. The system control data, which is typically encoded in selected horizontal lines of the vertical blanking interval of the transmitted television signal, usually includes such information as subscriber address codes, subscriber authorization codes, program codes and the like enabling the system operator to designate different levels of programming authorized for different individual subscribers. See, for example, U.S. Pat. No. 4,460,922, entitled Memory Select System for an STV Decoder, which describes the use of such system control data in detail.
A known technique for scrambling a subscription television signal contemplates inverting randomly selected fields of a broadcast television signal such that the reproduced video display on a standard television receiver is caused to alternate between normal and inverted or negative images. U.S. Pat. No. 4,222,068 to Thompson discloses a subscription television system exemplary of this type of video scrambling. In the Thompson system, a mode control signal in the form of an inverted polarity horizontal line is transmitted during each vertical blanking interval immediately preceding the transmission of each inverted video field to enable decoding thereof by authorized system subscribers. In particular, the decoder of an authorized subscriber is responsive to each transmitted mode control signal for re-inverting the immediately succeeding video field of the received television signal and is responsive to the absence of a mode control signal for developing the immediately succeeding video field in an unmodified form. As a result, the decoder is operable for unscrambling the transmitted television signal which may then be coupled to a standard television receiver for viewing.
As explained above, the mode control signal used in the Thompson system to identify an inverted video field consists of a selected horizontal line of the preceeding vertical blanking interval whose polarity has been inverted. Although this mode control signal is relatively easy to detect for providing increased confidence in proper decoder operation, its ease of detection also facilitates unauthorized decoding of the scrambled signal by a would-be pirate. That is, by simply detecting the polarity of the selected horizontal line comprising the mode control signal, a would-be pirate can quite easily duplicate the operation of an unauthorized decoder to unscramble the transmitted television signal.
While the prior art discloses numerous techniques for encrypting mode control signals of the type used in the Thompson system to deter the would-be pirate, such attempts have not proven altogether satisfactory. For example, U.S. Pat. No. 3,824,332 to Horowitz discloses a subscription television system wherein coding bursts are added to the composite television signal to indicate whether the subsequent video field is inverted. U.S. Pat. No. 4,068,264 to Pires discloses the use of randomly generated binary code number signals as mode control signals. U.S. Pat. No. 3,919,462 to Hartung et al discloses the use of a mode control signal which is coded in binary digital form as a series of pulses. Finally, U.S. Pat. No. 4,081,832 to Sherman discloses a television system wherein a decoder is operated for re-inverting video signals in response to information contained on a punch card inserted in a suitable card reader.
It is accordingly a primary object of the present invention to provide an improved technique for encrypting a mode control signal for use in a subscription television system.
It is a more specific object of the invention to provide a subscription television system characterized by a mode control signal which is encrypted such that the bits of the transmitted mode control signal define a predetermined inversion control code when processed according to a selected function with selected bits of the transmitted system control data.
It is a further object of the invention to provide a decoder for use in a subscription television system for recovering the predetermined inversion control code from the transmitted mode control signal and for enabling unscrambling of the accompanying television signal on a majority voting basis wherein unscrambling is enabled in response to the majority of bits of the recovered inversion control code coinciding with the corresponding bits of the predetermined inversion control code.
It is yet another object of the invention to provide a subscription television system wherein a transmitted mode control signal is characterized by a plurality of illusory data bits adapted to deter unauthorized decoding of subscription television programs.